1. Field of the Invention
The present invention relates to a method of driving a door of an automatic door assembly and more particularly, to a method of driving the door by a linear motor mounted within the automatic door assembly.
2. Description of the Prior Art
Automatic door assemblies having a linear motor as a prime mover are advantageous in that the linear motor can drive the door linearly without any special power transmission mechanism, the motor is simple and durable in construction, and it can be manufactured less costly. However, they also have a disadvantage in that the linear motor, which has a relatively reduced stroke, is actuated only for a period during which time the slip of the linear motor remains relatively large. Therefore, when the propulsion force of the linear motor is too small, the door is retarded and tends to stop before it reaches the end of its stroke or travel, and when the propulsion force is too great, the door is driven so rapidly that the door frame is subjected to the full impact of the moving door. In addition, since frictional resistance and inertia acting on the door vary with the structure and extent of movement of the door when the latter travels, it is extremely difficult to determine a propulsion force for the motor which prevents the door from stopping before it comes to the end of its stroke or from bumping against the door frame at the end of its travel or stroke. Various attempts have heretofore been made to stop the door exactly at the ends of the door stroke by retarding the door during its stroke, thereby preventing the door from striking the outer frame. One such attempt has been to reduce the speed of the door electrically by giving an opposite propulsion force to the linear motor during a final portion of the door stroke. However, this has led to a drawback in that various intricate control devices such as a speed detecting device and a position detecting device which must be adjusted precisely need to be added to the automatic door assembly. Another such attempt has been to provide a pair of cushioning devices such as self-returning type air cylinders at the ends of stroke of the movable door so as to dampen the door speed mechanically.
A problem with the automatic door assembly having the cushioning devices is that an additional propulsion force must be applied at the ends of the door stroke to overcome the reaction force of the cushioning devices. Furthermore, a common difficulty experienced with these prior proposals is that the door cannot be moved smoothly over its entire stroke and is subjected to stresses since the door is forcibly stopped at the ends of the stroke.